Abstract: A research for the optimization of oil recovery rate and water injection method was conducted with numerical simulation and field practice so as to improve the accuracy of numerical simulation of fractured reservoirs and provide effective water injection strategies.In the study,a buried hill reservoir in JZ25-1S oilfield in the Bohai Bay Basin,a large fractured reservoir, is taken as a study object. With the use of digital core technology and multiphase flow simulation, the characteristics of micropores and throats in the upper and lower semi-weathered crust and the characteristics of relative permeability curve are defined. The results indicate that development degree of microfractures plays an important role in controlling the shapes of relative permeability curve and the capillary curve of matrix block; the more developed the microfractures in the core, the higher the oil displacement efficiency, and the more conducive to matrix imbibition; the length and frequency of microfractures follow a power-law distribution, with a power-law index of approximately 1.5; in the initial stage of development, the production are mainly from the fracture system, while in the middle and later stages of development, the production from the matrix system gradually increases; the cumulative production contribution ratio of the fracture system to the matrix system is approximately 2∶1;the recovery rate is expected up by 4.5 percentage points through the optimization of the oil recovery rate in the initial stage and utilization of cyclic water injection during the water-cut rising stage. This study provides important guidance for development strategies and water injection schemes for fractured reservoirs.

Key words: fractured reservoirs, micro-fractures, relative permeability curve, capillary force curve, oil recovery rate, cyclic water injection, digital core